Unsaturated polyesters derived from the esterification of polyepoxides with ethylenically unsaturated carboxylic acids are known. See, for example, U.S. Pat. No. 3,408,422. These polyesters, and particularly when combined with unsaturated monomers such as styrene, have limited shelf life, i.e., after storage at room temperature for some time they undergo premature gelation to form products having little or no utility. Efforts have been made to prevent the gelation both during the esterification step and during storage of the polyester product, by the addition of known stabilizing materials, but the results have not been too satisfactory. In many cases, the stabilizing materials have added undesirable color to the product and/or have interfered with the cure of the polyester. It would be very desirable to have a stabilizing material which would not affect color, would not interfere with the cure and could be employed in small amounts. A very good stabilizer includes the dialkylhydroxylamines disclosed in U.S. Pat. No. 3,408,422. It will be appreciated that the catalysts employed in U.S. Pat. No. 3,408,422 are the onium salts, which are believed to be excellent esterification catalysts; however, when an organic phosphine is utilized as the esterification catalyst, the resulting polyesters are not as stable as desired for some applications. In other words, when an organic phosphine is used as an esterification catalyst, particularly if the resulting polyester is modified by further reaction with a dicarboxylic acid anhydride, the polyester composition tends toward premature gelation. It would therefore be highly desirable to improve the stability of the phosphine-catalyzed polyester processes and the resulting polyester compositions.